This invention relates to radar altimeter systems.
Radar altimeters are commonly used in aircraft to provide information to the pilot as the aircraft's height above ground. It has also been proposed to use the output from a radar altimeter to provide information to terrain reference navigation (TRN) systems. TRN systems have a stored digital database of ground contours over which the aircraft is to fly and correlate height information from the radar altimeter with the database to derive information as to the position of the aircraft in terms of, for example, latitude and longitude.
Radar altimeters take one of two different forms: one uses short pulses of radar emissions, and the other uses a continuous wave, frequency modulated (FMCW) radar emission. The pulsed form of radar has an advantage over continuous wave equipment, in that it is more readily responsive to relatively small radar targets, such as low density woods, small hills and so on. This can be important in low flying aircraft, since the pilot needs information about his height above any possible collision object on the ground, not just information about height above the ground itself. Pulsed equipment can also be rendered less susceptible to detection from outside the aircraft, making the aircraft itself less easy to detect by hostile observers.
The inherent sensitivity of pulsed radar altimeters, however, makes them less suitable for use in TRN systems since the database in such systems is usually of only the more general features such as ground contours. Correlation of the output of a pulsed radar altimeter with such a database would be more difficult than with a less sensitive continuous wave radar altimeter.
One solution to this is for the aircraft to have two altimeters, one of the continuous wave type, providing its output to the TRN system, and the other of the pulsed type, providing its output to the pilot's display. This, however, doubles the cost of acquiring, of installing and of maintaining the radar altimeters. Because each altimeter will require its own antenna this may present problems of installation. The increased weight, power consumption, heat dissipation and risk of external detection caused by this duplication can also be a significant problem. Also, FMCW systems are less readily adjustable according to the nature of the ground surface over which they are being used.